1. Field of the Invention
The present invention relates to a thermal head that is incorporated into a printer mechanism, a thermal head manufacturing method, and a thermal printer.
2. Description of the Related Art
As thermal heads of related art designed for incorporation into a printer mechanism, those as set forth hereunder have been known.
For example, disclosed in Japanese Unexamined Patent Publication JP-A 61-154954 is a thin-film type thermal head constructed by laminating, on a substrate, a heater element, a wiring conductor layer, and an abrasion-resistant protective film successively in this order. In this thermal head, the abrasion-resistant protective film is designed to have a double-layer structure including a first protective film made of an SiN-based material which exhibits high insulation resistance, and a second protective film made of an SiC-based material or TiC-based material which exhibits low insulation resistance. The second protective film, which is formed on the first protective film, is connected to a common electrode of the wiring conductor layer.
Disclosed in Japanese Unexamined Patent Publication JP-A 02-266959 is a thermal head composed of: a substrate; a heater element and an electrode formed on the substrate; an insulating layer made of SiO2 for covering the heater element and the electrode; an electrically conductive layer made e.g. of NiCr, Au, or Al that is formed on the insulating layer; and a protective film made of an SiN-based, SiC-based, or TaO-based material that is formed on the electrically conductive layer. In this thermal head, the electrically conductive layer is kept at the same potential as a recording power supply voltage, and the protective film is connected to ground.
Disclosed in Japanese Unexamined Patent Publication JP-A 11-277782 is a thermal head which has, in order to protect a heater element, at least a single protective film formed of a carbon protective film made of a material which is predominantly composed of carbon. The content of oxygen present at the interface between the carbon protective film and a layer formed thereunder is set at 20 atm % or below.
Also disclosed therein is a thermal head which has, as the lower layer formed under the carbon protective film, an intermediate protective film. Moreover, as a layer formed under the intermediate protective film, a lower protective film is provided that is made of a ceramics material such as silicon nitride, silicon carbide, or SiAlON. The intermediate protective film is made of a material which is predominantly composed at least of one kind of metal elements selected from the group consisting of metals of the 4A, 5A, and 6A groups, Si, and Ge.
Disclosed in Japanese Unexamined Patent Publication JP-A 2000-177158 is a thermal head composed of: a substrate; a heater element formed on the substrate; an electrode layer connected to the heater element; a protective film for covering at least a heating portion of the heater element; and at least two pieces of anti-static layers formed on the protective film. The two (at least) anti-static layers are made of a metal element selected from the 3A through 7A groups, silicon, and oxygen. In this specification, Ta and Nb are suggested as the examples of the metal element.
Moreover, the two (at least) anti-static layers include an insulating anti-static layer formed in contact with the protective film, and a conductive anti-static layer formed as an uppermost layer. The surface resistance of the insulating anti-static layer is set at 1×104 Ω or above, whereas the surface resistance of the conductive anti-static layer is set at less than 1×104 Ω.
Also disclosed therein is a thermal head manufacturing method whereby at least two pieces of the anti-static layers are formed by making changes to the quantity of oxygen supply on the protective film.
Disclosed in Japanese Unexamined Patent Publication JP-A 2001-47652 is a thermal head composed of a discrete electrode layer, a common electrode layer, and a heater element that are all arranged on a substrate. Moreover, on the heater element is formed an insulating protective film, and on the insulating protective film is formed a conductive protective film which is higher in thermal conductivity than the insulating protective film. The conductive protective film constitutes a stacked-layer structure such that the conductive protective film makes direct contact with the common electrode layer at least in the region corresponding to the entire range of effective print width.
Moreover, the conductive protective film is made of a thick-film conductive paste which contains at least ruthenium, the sheet resistance of which is adjusted to fall in a range from 0.5 to 10 MΩ/square.
Disclosed in Japanese Unexamined Patent Publication JP-A 2001-270141 is a thermal head composed of: a substrate; a heater element formed on the substrate; a common electrode layer and a discrete electrode layer connected to the heater element; a protective film formed so as to cover the entire top surface of the substrate; and an anti-static conductive layer formed on the protective film. In this thermal head, part of the protective film located on the common electrode layer is removed, so that the common electrode layer and the conductive layer can make interface-contact with each other in the protective film-free region.
Incidentally, regarding a thermal head such as that in which a conductive protective film is formed on an insulating protective film and the conductive protective film is directly connected to a common electrode layer, the inventors of the present application have found the following problem to be addressed. That is, in a process for cleaning the thermal head, the conductive protective film and the common electrode layer are immersed in a cleaning solution at the same time. In this case, a so-called battery effect takes place between the common electrode layer and the conductive protective film, causing Al that constitutes the discrete electrode layer and the common electrode layer to dissolve into Al ions in the cleaning solution. As a result, the electrode layers could suffer from corrosion (galvanic corrosion)